raindrops - real-time stats for preforking Rack servers

raindrops is a real-time stats toolkit to show statistics for Rack HTTP
servers. It is designed for preforking servers such as unicorn, but should
support any Rack HTTP server on platforms supporting POSIX shared memory.
It may also be used as a generic scoreboard for sharing atomic counters
across multiple processes.

Features

counters are shared across all forked children and lock-free

counters are kept on separate cache lines to reduce contention under SMP

Inline patches (from "git format-patch") to the mailing list are preferred
because they allow code review and comments in the reply to the patch.

We will adhere to mostly the same conventions for patch submissions as git
itself. See the Documentation/SubmittingPatches document distributed with
git on on patch submission guidelines to follow. Just don't email the
git mailing list or maintainer with raindrops patches.